1. Field of the Invention
The present invention relates to a heating device for heating a member to be heated, by electromagnetic induction, and an image forming apparatus.
2. Description of the Related Art
Conventionally, there has been known a heating device for heating a member to be heated (hereinafter referred to as “to-be-heated member”) by electromagnetic induction. For example, in an image forming apparatus, such as a copying machine or a printer, such a heating device heats a metal roller or a metal belt, which is a to-be-heated member, by electromagnetic induction, and fixes a toner image formed on a sheet using the heat of the heated to-be-heated member.
There has also been known an image forming apparatus of this type, which detects a change in the state of a metal belt as the to-be-heated member, such as a damage, as disclosed in Japanese Patent Laid-Open Publication No. 2007-328159.
FIG. 13 schematically shows a fixing device as a heating device for the image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-328159. The fixing device has an inlet upper roller 6 and an exit upper roller 7 arranged on upstream and downstream sides, respectively, in a manner spaced from each other. A fixing belt 2, which is an endless metal belt, is wound and stretched between the inlet and exit upper rollers 6 and 7. A nip pad 8 for applying pressure to a sheet, and a temperature-detecting thermistor 4 are arranged inside the fixing belt 2. An inlet lower roller 10 and an exit lower roller 11 are arranged on the upstream and downstream sides, respectively, in a manner spaced from each other, and an endless pressing belt 9 is wound and stretched between the inlet and exit lower rollers 10 and 11. A nip pad 12 is disposed inside the pressing belt 9. An antenna 3 for detecting magnetic flux is disposed inside the fixing belt 2 at a location opposite to an induction heating coil 1 with the fixing belt 2 positioned therebetween. The antenna 3 is connected to an energization inhibition circuit.
The magnetic flux entering the antenna 3 varies depending on the state of the fixing belt 2, and therefore, the image forming apparatus is configured to detect the state of the fixing belt 2 according to the magnetic flux entering the antenna 3 and make the energization inhibition circuit operable to stop the operation of the induction heating coil 1 if there is abnormality in the state of the fixing belt 2.
In the configuration disclosed in Japanese Patent Laid-Open Publication No. 2007-328159, as shown in FIG. 14A, the antenna 3 extends in the direction of width of the fixing belt 2, and by making use of the phenomenon that the amount of magnetic flux entering the antenna 3 is changed by abnormality of the fixing belt 2 from that in a normal state thereof, detects a change in the state of the fixing belt 2, that is, belt abnormality, based on a change in the magnetic flux. Therefore, to detect a belt abnormality which is small when the width of the fixing belt 2 is considered, it is required to set a small reference threshold value for determining the belt is abnormal.
However, even if the fixing belt 2 is in the normal state, the amount of magnetic flux entering the antenna 3 does not become equal to 0, and moreover even when the fixing belt 2 is normal, the amount of magnetic flux entering the antenna 3 varies with the strength of induction heating or the like. Therefore, to prevent the fixing belt 2 in the normal state from being erroneously detected to be abnormal, it is necessary to set a higher threshold value. This brings about the problem that it is difficult to detect a belt abnormality occurring in a narrow range.
To solve such a problem, the present assignee has studied a configuration shown in FIG. 14B in which the antenna is twisted at a central portion in an extending direction thereof, and inverts the polarity of output, to thereby cause generated voltages to cancel each other out. According to this configuration, it is considered that the amount of magnetic flux entering the antenna 3 increases only in an area where abnormality of the fixing belt 2 has occurred, whereby even a relatively small abnormality can be detected.
However, as shown in FIG. 14A, it is also assumed that belt abnormality occurs in an area X which is within a range equidistant from the central position of the antenna 3 in the extending direction of the antenna 3. In such a case, no difference is caused in the amount of magnetic flux entering the antenna 3 between the back side (upper portion in FIG. 14A) and the front side (lower portion in FIG. 14A) of the antenna 3, whereby it is predicted that abnormality of the fixing belt 2 cannot be detected. More specifically, in the case where the antenna configuration shown in FIG. 14B is employed, it is difficult to detect a belt abnormality which occurs in an area extending equally in opposite directions along the width of the belt from the central portion of the antenna 3 on opposite sides of which the polarity is inverted therebetween, which adversely affects the accuracy of belt abnormality detection.